CMSC 611: Advanced Computer Architecture Performance Some material adapted from Mohamed Younis, UMBC CMSC 611 Spr 2003 course slides Some material adapted from Hennessy & Patterson / © 2003 Elsevier Science

Moore’s Law Transistors double every year –Revised to every two years –Sometimes revised to performance instead of transistors (Wgsimon - Wikipedia) (Source: Intel) 2

Criteria of performance evaluation differs among users and designers Defining Performance Performance means different things to different people, therefore its assessment is subtle Analogy from the airlines industry: How to measure performance for an airplane? –Cruising speed (How fast it gets to the destination) –Flight range(How far it can reach) –Passenger capacity(How many passengers it can carry) 3

Performance Metrics Response (execution) time: –The time between the start and the completion of a task –Measures user perception of the system speed –Common in reactive and time critical systems –Single-user computer Throughput: –The total number of tasks done in a given time –Relevant to batch processing (billing, credit card processing) –Also many-user services (web servers) Power: –Power consumed or battery life –Especially relevant for mobile 4

Response-time Metric Maximizing performance means minimizing response (execution) time 5

Response-time Metric Performance of Processor P1 is better than P2 if –For a given work load L –P1 takes less time to execute L than P2 6

Response-time Metric Relative performance captures the performance ratio –For the same work load 7

Designer’s Performance Metrics Users and designers measure performance using different metrics –Users: quotable metrics (GHz) –Designers: program execution Designer focuses on reducing the clock cycle time and the number of cycles per program Many techniques to decrease the number of clock cycles also increase the clock cycle time or the average number of cycles per instruction (CPI) 8

A program runs in 10 seconds on a computer “A” with a 2 GHz clock. We desire a faster computer “B” that could run the program in 6 seconds. The designer has determined that a substantial increase in the clock speed is possible, however it would cause computer “B” to require 1.2 times as many clock cycles as computer “A”. What should be the clock rate of computer “B”? Example Why would this happen? –Maybe one operation takes one full clock cycle at 2 GHz (1/2 GHz = 0.5 ns) –All the rest take less than one cycle –Split this operation across multiple cycles –Can now increase clock rate, but also increase total cycles 9

A program runs in 10 seconds on a computer “A” with a 2 GHz clock. We desire a faster computer “B” that could run the program in 6 seconds. The designer has determined that a substantial increase in the clock speed is possible, however it would cause computer “B” to require 1.2 times as many clock cycles as computer “A”. What should be the clock rate of computer “B”? To get the clock rate of the faster computer, we use the same formula Example 10